In recent years, the technique for attaining ultra-high vacuum or the technique for producing ultra-high clean reduced pressure atmosphere by introducing a gas at a low flow rate into vacuum chamber is becoming increasingly important. These techniques are applied for the research of material characteristics, for the formation of various types of thin film, for the manufacture of semiconductor devices, etc. As the result, higher degree of vacuum is attained, while there is a strong demand on the reduced atmosphere, where the intermingling of impure elements and impure molecules could be reduced to utmost extent.
For example, in the manufacture of semiconductor devices, the dimensions of unit elements are being reduced year by year to attain higher integration of integrated circuit. Fervent research and development activities are carried out for the practical application of semiconductor devices having a dimension of 1 .mu.m to submicron, or 0.5 .mu.m or less.
Such semiconductor devices are manufactured by repeating the process for forming thin film and the etching process of the film thus formed into the specified circuit pattern. Usually, such processes are performed in ultra-high vacuum conditions or in reduced pressure atmospheres with the specified gas by placing the silicon wafers into the vacuum chamber. If the impurities are intermingled during these processes, the quality of thin film may be reduced or the precision fabrication may not be achieved. This is the reason why an ultra-high vacuum and an ultra-high clean reduced pressure atmosphere is wanted.
One of the major reasons hindering the actualization of an ultra-high vacuum and an ultra-high clean reduced pressure atmosphere has been the gas released from the surface of stainless steel widely used for the chamber of the gas pipe. Above all, the source of the worst contamination has been the moisture adsorbed on the surface, which is released under vacuum or reduced pressure atmosphere.
FIG. 9 is a graphic representation showing the relation between total leakage of the system, including the gas piping system and reaction chamber in each apparatus (the sum of gas quantity released from inner surface of the piping system and reaction chamber with the external leakage), and gas contamination. It is assumed that the original gas does not contain the impurities. The lines in the diagram indicate the results when the values are changed with gas flow rate as a parameter. Naturally, the lower the gas flow rate is, the more the impurities concentration increases as the influence of the released gas from the inner surface becomes conspicuous.
In the semiconductor manufacturing process, there is a trend to increasingly reduce the gas flow rate in order to attain a process of higher accuracy by opening and filling the holes with a high aspect ratio. For example, it is now normal to use the flow rate of several tens of cc/min. or less for the process of manufacturing ULSI of submicron order. Suppose that the flow rate is 10 cc/min. and that the total leakage of the system is 10.sup.-3 to 10.sup.-6 Torr l/sec. such as the apparatus currently in use, the purity of the gas is 1% to 10% ppm, this being far from the high clean process.
The present inventors have invented the ultra-high clean gas supply system and have succeeded in reducing the leakage from outside the system to less than 1.times.10.sup.-11 Torr l/sec. which is the detection limit of the detectors presently in use. However, the concentration of the impurities in the reduce pressure atmosphere could not be reduced due to the leakage from inside the system or due to the components of the related gas from the stainless steel surface. In the case of stainless steel the minimum value of the surface released gas quantity as obtained by the surface treatment in the ultra-high vacuum technique at present is 1.times.10.sup.-11 Torr l/sec. cm.sup.2. Suppose that the surface area exposed to the interior of the chamber is estimated to the minimum, e.g. to 1 m.sup.2, the total leakage is 1.times.10.sup.-7 Torr l/sec. This means that only the gas with a purity of about 1 ppm can be obtained with a gas flow rate of 10 cc/min. The purity is doubtlessly decreased when gas flow rate is lowered further.
In order to decrease the released gas from the inner surface of the chamber to 1.times.10-11 Torr l/sec., i.e. to the same level as the external leakage of the total system, it is necessary to set the released gas from the surface of stainless steel to less than 1.times.10-15 Torr l/sec. cm.sup.2. As a result, there is a strong demand for a better processing technique in order for the surface of stainless steel to have a lower gas release.
In the semiconductor manufacturing process, a wide variety of gas is used from relatively stable common gases (such as O.sub.2, N.sub.2, Ar, H.sub.2, He) to special gases having reactivity, corrosive property and toxicity. As the material for the piping and chamber for these gases stainless steel is normally used because of its higher reactivity, corrosion resistance, high strength, easy secondary fabrication, weldability and easy polishing of its inner surface.
Stainless steel shows excellent corrosion-resistant property in a dried gas atmosphere Among the special gases, however, there are boron trichloride (BCl.sub.3) or boron trifluoride (BF.sub.3), which generates a high corrosive property by generating hydrochloric acid or hydrofluoric acid through hydrolysis when moisture exists in the atmosphere. Thus, stainless steel is easily corroded when moisture exists in the gas atmosphere containing BCl.sub.3 or BF.sub.3. Therefore, anti-corrosion processing is indispensable after surface polishing of stainless steel.
For anti-corrosion processing, there are several methods, one of which is Ni-W-P coating (clean escorting method) to coat the highly corrosion-resistant metal onto the stainless steel. There are some problems with this method because cracking and pinholes often occur and the adsorbed moisture on the inner surface or the residual solution components increase because wet type metal plating is employed. There is also another anti-corrosion processing method which is a passivation treatment to produce an oxide film onto the metal surface. Stainless steel is passivated when it is immersed in a solution containing a sufficient quantity of an oxidizer. In this method, stainless steel is usually immersed in a nitric acid solution at a normal temperature or a little higher, thus the passivation treatment is performed. However, this method is also of a wet type, and the residues of moisture and the processing solution remain on the inner surface of the piping and the chamber. In the methods as described above, the existence of moisture adsorbed on the inner surface of the piping procedures severe damage to the stainless steel when a chlorine or fluorine type gas is introduced.
Therefore, it is very important for the ultra-high vacuum technique or in the semiconductor manufacturing process to fabricate the chamber or the gas piping system with stainless steel having a passivated film, which is not easily damaged by corrosive gas and which occludes or adsorbs less moisture.
For example, in the passivation treatment of stainless steel pipe, a passivated film having excellent degassing property is obtained when heating and oxidation are performed in a highly clean atmosphere with moisture content of less than 10 ppb.
FIG. 10 summarizes the changes of moisture contained in the purge gas when the stainless steel pipes with different internal process conditions are purged at normal temperature. In the experiment, argon gas was passed at a flow rate of 1.2 l/min. through 3/8" stainless steel pipe having a total length of 2 m, and the moisture content in argon gas at the outlet was determined by APIMS (atmospheric pressure ionization mass spectrometer).
The stainless steel pipes under test are divided into three types: (A) Stainless steel pipe with an inner surface processed by electrolytic polishing; (B) Stainless steel pipe with an inner surface processed by passivation treatment with nitric acid after electrolytic polishing; (C) Stainless steel pipe, on which the passivated film is formed by heating oxidation in a highly clean and dry atmosphere after electrolytic polishing. In FIG. 10, these are respectively represented by the curves A, B and C. The experiment was performed after leaving each of these stainless steel pipes in a clean room maintained at a relative humidity of 50% and at a temperature of 20.degree. C. for about one week.
As is evident from the curves A and B, a large quantity of moisture was detected from the electropolished pipe (A) and the electropolished pipe having a passivation treatment with nitric acid (B). After the gas was passed for about one hour, a moisture content of 68 ppb was detected in A and 36 ppb in B. Moisture content did not decrease after 2 hours, showing 41 ppb and 27 ppb in A and B respectively. In contrast, the moisture content decreased to 7 ppb within 5 minutes after the gas was passed through the pipe (C) with the passivation film formed in a highly clean and dry atmosphere After 15 minutes, it decreased to less than the background level of 3 ppb. Thus, it was demonstrated that (C) has excellent degassing property to adsorption gas.
However, in order to attain an ultra-high clean oxidation atmosphere with a moisture content of less than 10 ppb in order to produce the stainless steel pipe similar to (C) in FIG. 10, it is essential to have high-grade condition control. This involves higher cost and lower production efficiency and is no suitable for mass production. In other words, it is impossible to attain an ultra-high clean oxidation atmosphere by the metal oxidation apparatus and metal oxidation method as conventionally employed.
Particularly, in the stainless steel pipe or the stainless steel pipe having the curved portion with a smaller inner diameter such as 1/4", 3/8" and 1/2", gas is very likely to stagnate, and oxidation treatment is performed with the inside of the stainless steel pipe exposed to atmospheric air, resulting in contamination. This makes it impossible to form the passivation film of good quality having superb corrosion-resistant property and with lesser moisture occlusion and adsorption. Because the outer surface of stainless steel pipe is not directly related with the supply o ultra-high purity gas, the surface becomes contaminated after oxidation treatment due to roughness and dirtiness of the surface. The oxidation of the outer surface of stainless steel pipe results in the problems such as poor external appearance or the generation of particles when pipes are installed in a clean room.
Therefore, there have been strong demands on the establishment of a mass production technique for the passivation treatment for stainless steel pipe in order that the passivation film is formed to provide the inner surface with excellent corrosion-resistant property and to occlude or adsorb moisture in lesser content and that the outer surface is not oxidized.
The object of the present invention is to solve these problems by offering a metal oxidation treatment apparatus and a metal oxidation treatment method, by which the contamination caused by the released gas or impurities such as moisture from the oxidized surface of stainless steel pipe having a curved portion is reduced and the stainless steel pipe for a ultra-high vacuum and an ultra-high clean reduced pressure apparatus and for a gas supply system having an excellent corrosion-resistant property can be produced in large quantities.
Another object of this invention is to offer a metal oxidation treatment apparatus capable of self-cleaning and self-maintenance in addition to the above object.